Django中的从属字段

Question

I'm in the process of designing models for a Django app and I can't quite work out how to approach a problem.

I have 3 models. Server , Module , and Device .

Server is a server that a module could run on. Module runs on a server, and contains devices.

The problem with Devices is that the name is always a field, but the attributes could change.

I considered just putting a JSON value in attributes but I couldn't figure out how to do that with Django admin.

Basically the user will enter a name for the device and depending on which module it is attached to, show various different fields to fill in.

For example:

module rfm_ninjablock:
Device fields would be "name" and "code"

module LimitlessLED
Device fields would be "name", "controller", "group" and "LightID"

Is there a straightforward way to do it or am I going about it the wrong way?

Answer 1

One way to do this is to make a fourth model: DeviceAttribute .

• Each Device has a name and has zero or more DeviceAttribute s.

• A DeviceAttribute has two fields: key and value .

Alternatively, you could use the JSON you suggested, but you wouldn't be able to query on it or update it as easily. It's a bit of an anti-pattern.

UPDATE: If you have several "classes" of Device s, then rather than having arbitrary attributes, you can achieve type-safety by subclassing your models.

There are several ways to do this. I have often used the multi-table approach.

It creates a OneToOneField between DeviceSubclassA and Device , and between DeviceSubclassB and Device .

You can add certain fields to DeviceSubclassA , and different fields to DeviceSubclassB . The common name attribute is on the Device parent model.